A communications system comprises a transmitter coupled to a receiver via a communication channel. The system functions to send and receive information signals. In the course of communicating information signals through the channel, such signals may be corrupted by channel noise and other error sources (e.g., fading) associated with the channel. In a digital communication system, these error sources may be responsible for, among other things, altering the content of the information received. To help prevent such error sources from corrupting received information signals, any of a number of channel coding techniques may be utilized in system transmitter and receiver design.
Generally, a channel coding technique helps mitigate the effects of error sources by introducing so-called "redundancy" to the information to be communicated. Because of this redundancy, the likelihood that noise will corrupt communicated information is reduced. Some communication errors may be completely eliminated.
Some communication systems transmit more than one information signal over a single channel. This may be accomplished with use of a conventional multiplexer combining individual information signals into a combined signal for transmission over the channel. To mitigate the effects of channel error sources on the communication of the combined information signal, a channel coder is applied to the combined signal. The coded result may then be provided to, for example, a conventional interleaver to help reduce the effects of fading.
The use of a single channel coder for a combined information signal can adequately guard against channel errors, but it can also be wasteful of bandwidth. This is because each of the individual information signals which form the combined information signal may have its own distinct level of robustness to channel errors.
For example, an audio signal and a separate information signal representing alphanumeric characters may be multiplexed into a combined information signal for transmission over a radio channel. Such an audio signal might be Beethoven's 5th symphony, while the alphanumeric signal might identify or advertise (in advance) the transmission of music to be played, to wit, "Beethoven's 5th Symphony." Such an advertisement might be displayed to an interested listener.
Channel errors which :result from transmission of such a combined signal can manifest themselves differently in each received component signal. A channel error affecting the audio portion of the combined information signal might manifest itself as an almost inaudible click, pop, or momentary signal drop-out. Such a channel error may not be very bothersome or even noticeable, for that matter, to a listener of the audio program. Moreover, the system receiver may be able to take any one of several possible remedial actions to mitigate the effects of such errors on the audio signal. In these ways, the audio signal may be thought of as "robust" to channel errors.
However, the alphanumeric signal is not nearly so robust. In fact, when channel errors affect the alphanumeric information signal, the result can be more nearly catastrophic. This is because unlike errors in the audio portion of the combined signal, errors in the alphanumeric signal may manifest themselves in a fashion which cannot be remedied or ignored, to wit, "Bejth6oen*s 17h Sym9hehy."
Thus, it may be appropriate to provide channel coding on the combined signal sufficient to protect the alphanumeric component. However, such degree of coding would also be applied to the audio component of the signal--a degree of coding unnecessary to protect the audio signal from the effects of channel errors. This "overcoding" of the audio component is wasteful of channel bandwidth since signal redundancy provided by channel coding "costs" bandwidth which might be used for other purposes.